In order to keep up with rapid increases in data traffic as typified by the Internet and in demands for multimedia communications combining image, voice, and data, the speed and capacity of transmission paths and communication nodes that form networks are being improved, and optical communication systems using optical fibers and optical signals are being brought into use. In addition, as an alternative to conventional communication equipment in which optical signals are processed through optical-to-electrical conversion, optical signal processors such as the optical cross-connect (referred to as an OXC below) and optical add drop multiplexer (referred to as an OADM below), in which switching operations such as transmission path switching and circuit switching are carried out without such conversion, are under consideration for practical use.
The OXCs and OADMs mentioned above are configured by selectively using optical amplifiers, optical couplers, optical isolators and other optical components as required and combining (interconnecting) them with optical fibers and connectors. As can typically be seen in optical switches and other optical devices, it is difficult to increase their capacity as matters now stand, so high-capacity optical switching systems are generally implemented by combining a number of low-capacity optical components. A higher-capacity optical switch, for example, can be implemented by multistage-connecting low-capacity optical switches, such as 2×2 or 8×8 optical switches that are already in commercial use.
As described above, an optical switching system is implemented by interconnecting a number of optical components and optical fibers with connectors and splices, so optical signals passing through the system suffer degradation due to optical loss in the components, and to various conditions at the connecting points, such as dirt, axial deviation, and open connection ends, which may give rise to the departure of part of an optical signal from the proper course. In particular, reflection in the direction opposite to the proper direction of propagation causes degradation of the optical signal.
Some optical signal processors and optical components that detect optical reflection have already been introduced. optical switches such as the one disclosed in JP-A-358261/2000 have been suggested, which comprises a reflected light detector at the input terminal thereof and a reflector at the output terminal thereof, and checks internal paths by confirming that an optical signal input from the input terminal is reflected back to the input terminal.
In optical signal processors configured by combining a plurality of optical components such as optical amplifiers, optical switches, optical couplers, and optical isolators as mentioned above, light reflected at a connection point of another optical component, resulting in multiply reflected light.
This multiply reflected light becomes a delayed version of the intended optical signal, so it interferes with the intended optical signal (causing degradation of the optical signal). Recent studies by the present inventor(s) have resulted in the discovery that degradation of optical signals caused by such multiply reflected light has a major effect on the operation of optical signal processors configured by combining a plurality of optical components.
More specifically, it was discovered that, in the optical switching system 300 in FIG. 2, when an optical (digital) signal 370 transmitted through optical fibers 310-1 to 310-N proceeds from an input port 330-N to an output port 340-N, multiply reflected light 375 that has been delayed at a reflecting point 1 indicated by reference numeral 350 and a reflecting point 2 indicated by reference numeral 360 may superimpose itself on the optical signal 370, causing coherent crosstalk, or interference between the optical signal and the multiply reflected light may form a resonator that is not actually present in the system. If a wavelength multiplexed signal is processed optically in an optical signal processor configured by combining a plurality of optical components, various types of optical degradation due to multiply reflected light may occur on a random basis: for example, (1) wavelength-dependent variations in optical-loss characteristics, (2) occurrence of signal amplitude noise due to wavelength fluctuations of an intended signal, and (3) wavelength dispersion. It has been found that these effects have a major effect on the operation of the system.
Therefore, practical utilization of an optical signal processor configured by combining a plurality of optical components requires configurations and methods by which reflected light arising in the processor during assembly, installation, or operation thereof can be detected reliably and immediately to enable alteration of optical signal paths and recovery actions (maintenance) such as replacement and repair of components, thereby improving the reliability, availability, and serviceability of the processor.
The document mentioned above describes a configuration for detecting singly reflected light, but it does not provide configurations and methods for implementing systems that address the problems of multiply reflected light in an optical signal processor configured by combining a plurality of optical components.